The present invention relates to a heat exchanger for cooling solid substance-containing gases, particularly gases discharged from a carbon gasification plant which has a plurality of heat exchange tubes through which gases pass and which tubes are connected at a gas inlet and gas outlet side to the tube bases.
During the carbon gasification at high temperatures a raw gas is generated which contains solid particles. These solid particles are combined from non-gasified carbon particles, carbon ash and slag particles. Before solid substance can be removed from gas this gas having the temperature higher than 1000.degree. C. must be firstly cooled down. Cooling of the gas is also required because usual processes for the desulfurizing of the gas have been performed at temperatures 150.degree. C.
In order to efficiently utilize heat produced by a hot raw gas stream the gas is conveyed through the heat exchanger where the gas transmits a part of its heat to a heat-receiving agent. In these conditions a tubular heat exchanger is preferable. Water is utilized as a heat-receiving agent, whereby steam is produced. The solid substance-containing gas is fed through the heat exchanger tubes whereas the boiler feed water is accommodated in the space between the outer sleeve of the heat exchanger and the outer side of the heat exchanger tubes. It is also possible to utilize wet steam as a heat-receiving agent and generate an overheated steam by heat exchange from the hot gas stream. It is also possible to utilize remaining gases or liquids other than water as heat-receiving agents.
During the feeding of gas through the heat exchanger tubes the speed of gas is selected so that the inner walls of the tubes are not contaminated by deposits and thus heat transmission ratios can not substantially change during the operation of the heat exchanger. Speeds between 20 and 50 m/s for adjusting desired heat transmission ratios and self-cleaning of the heat exchanger have been defined as suitable.
An inflow gas must at the gas inlet side of the heat exchanger, be distributed between individual heat exchange tubes. This gas is subject to direction changes and accelerations which can damage a uniform stream formation at the intake portions of the heat exchanger tubes. The length of the intake portion which is the extension between the inlet into the tube and the place of formation of a homogeneous tubular stream is the greater the more unfavorably-shaped is the tube intake portion. Unfavorable are abrupt cross-sections and changes in directions while favorable are channel shapes which ensure moderate accelerations and prevent transversal components in the stream.
When solid substance is present in the gas stream this leads to strong abrasion or erosion on the inner sides of the tubes, caused by unfavorable stream ratios due to non-laminar flows because of changes in directions, recirculation and flowing about corners and edges of the tubes. This indicates that particularly in the intake regions of the heat exchanger tubes, such cracks occur that can cause considerable reductions in the thickness of the tube in this region so that the operation of the heat exchanger becomes no longer reliable.
The tubes must be then closed which leads to blocking of efficient heat exchange surfaces and to increase of the stream speed in the operating heat exchanger tubes. Higher speed of the stream causes increase in wear and thereby shortening of the service life of the heat exchanger.
As soon as a greater part of the heat exchanger tubes is worn off as described above the heat exchanger must be provided with new tubing. It is however unsatisfactory that with tubes 7 m long wear takes place at the length of about 400 mm from the intake of the tube so that the remaining part of the tube is not damaged at all.
It has been proposed to extend intervals between replacements of the worn-off tubing with a new one by provision of insertion sleeves, also tubular, which have the outer diameter adjusted to the inner diameter of the heat exchanger tubes and which can be displaced along the place being damaged and thus cover the same. It has been discovered however that abrasion can occur immediately behind the end of the insertion sleeve due to sudden thickness enlargements, non-laminar stream with a vortex formation and recirculation. These insert sleeves can be eventually replaced with new insert sleeves which can be longer and which would overlap the entire area to be closed; these longer sleeves however have limited heat exchange properties.